Haemorrhagic Stroke

CT has always been considered useful in analysing cerebral haemorrhage, whether subarachnoid or intraparenchymal in location. Intracranial haemorrhage is apparent on CT immediately after the bleeding episode due to its high density relative to brain tissue. However, haemorrhage has a more complex appearance and explanation on MRI. An acute cerebral haemorrhage is primarily oxyhaemoglobin, a substance with no paramagnetic properties that behaves much like an aqueous solution, almost indistinguishable from an area of parenchymal ischaemia. In subacute haemorrhage, the parenchymal blood that was originally oxyhaemoglobin first turns into deoxyhaemoglobin, and then into intracellular and next into extracellular metahaemoglobin. These substances are paramagnetic and/or magnetically susceptible and therefore are able to influence relaxation times and change the MR signal in a somewhat predictable manner.

Recently released high field strength, high speed MR equipment and imaging sequences are particularly sensitive to magnetic susceptibility differences (e.g., echo planar imaging: EPI) has demonstrated that it is possible to clearly reveal even small areas of cerebral haemorrhage on MRI (15). Because of this ability of MRI to distinguish between non-haemorrhagic ischaemia and haematoma, MRI may replace CT at some time in the future where this is practicable.

However, the fact that high field strength, high speed gradient performance MRI is presently less widely available and more costly than CT determines that CT will still be the first diagnostic imaging examination carried out in such situations for the foreseeable future. On the other hand, emergency angiography should be reserved for cases of subarachnoid haemorrhage in order to search for causative pathology of stroke (i.e., haemorrhage) such as aneurysms or vascular malformations.